ARM: dts: stm32mp: Add OP-TEE reserved memory to SPL dtb

Add the "/reserved-memory/optee" node to the SPL devicetree. The
purpose is to allow configuring TZC regions when booting OP-TEE.

Signed-off-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>